begins

though.
He turned back to the notebook, beginning to despair. From the look of the skies, the first light of dawn was beginning to appear, and a fierce storm was in the offing. Once that storm broke, poetry would have to seek prosaic shelter.
Perhaps...
He was gripped by sudden excitement, and began scribbling hastily again. If he went back and changed...
Yes! Forget the eagle entirely. The bird was mostly a scavenger anyway. Concentrate on the statue.

Whose bold gaze and sure brow, through the perilous fight,
At the gates as we watched, were so gallantly standing?

Yes, that'd work! From there...
And the rockets' red glare, the bombs bursting in air,
He hadn't seen that himself, but the inhabitants had described it. Now...abit of fudging...

Gave proof through the night that our dame was still there.

He could get away with that, surely. True, the British had stopped the bombardment of the Capitol hours earlier, but they'd fired off an occasional rocket now and then. More for show than anything else, obviously, but that was a pedestrian matter that a poet could safely ignore.
Then...
Oh, those mindless soldiers and their imbecile Captain Houston! Key had the perfect closing couplet for the first stanza.

O say, does that star-spangled banner yet wave
O'er the land of the free and the home of the brave?

No banner, alas.
Key sighed. Nothing for it—once a poet begins with an image, he has to remain true to the thing, bloody